Text Manipulation Script

I need a text manipulation script .Here is the logic what I want; I have a
text and records as like;

A1
B1
C1
D1
E1
A2
B2
C2
D2
E2
A3
B3
C3
D3
E3
A4
so I want to create again text file between records A1-->A2 >file1.txt
then A2--->A3> file2.txt then A3--->A4 > file3.txt so this means at the
end I have got 3 files as includes the records.